He was previously paired with Hoon Thien How, with whom he won the World Junior Championships in 2004 and a silver medal at the 2006 Asian Badminton Championships.

Nearing the Doha XV Asian Games in 2006, Rexy Mainaky (the Malaysian doubles coach) decided to split them up and partner Tan Boon Heong with Koo Kien Keat instead. This move, nevertheless, proved to be spot-on as this pair went on to become the Asian Games champion, winning the gold medal in their maiden outing by defeating the then Chinese world champions, Cai Yun-Fu Haifeng, in the quarterfinals, Indonesia’s Markis Kido-Hendra Setiawan in the semifinals, and finally Luluk Hadiyanto-Alvent Yulianto Chandra, also from Indonesia, in the finals. They are the youngest men double to win gold medal at Asian Games at the age of 21 and 19 respectively.

The BWF Superseries, launched on 14 December 2006 and implemented in 2007, is a series of elite badminton tournaments, sanctioned by Badminton World Federation (BWF). BWF Superseries has two levels: Superseries and Superseries Premier. A season of Superseries features twelve tournaments around the world, which introduced since 2011, with successful players invited to the Superseries Finals held at the year end.

2007 was the best year for Koo and Tan. They became the first qualifiers to win the Super Series at the Malaysian Open. They also became the World No.1 that year. They won their first All England Super Series after beating Chinese pair, Cai Yun and Fu Haifeng in straight games.

At the 2009 Japan Super Series, Boon Heong set the world record for badminton smashes at 421 km/h. This was done under lab conditions and recorded by Yonex representatives, and not in an official match. Later, two won the bronze medal at the 2009 World Championships

At the 2010 BWF World Championships, Boon Heong and partner Koo Kien Keat launched themselves into the semifinals after beating Korean rivals Lee Yong Dae and Jung Jae Sung. In the semifinals they defeated China’s Guo Zhendong and Xu Chen 21-14, 21-18. Tan Boon Heong and Koo Kien Keat wrote their names in the record books, being the first Malaysian pair to enter a World Championships Final in 13 years. In the finals they played China’s Cai Yun and Fu Haifeng and lost 21-18, 18-21, 14-21. The year 2010 was the last time Koo and Tan ranked World No.1.

At the 2011 All England Super Series Premier, Boon Heong and Kien Keat defeated 2008 Olympic champions Markis Kido and Hendra Setiawan in the quarterfinals. They then defeated World champions Cai Yun and Fu Haifeng 21-11, 23-21. They lost to Danes and world no.1 Mathias Boe and Carsten Mogensen 21-15, 18-21, 18-21. In doing so, they failed to win their 2nd All England title.

They suffered a lot of early round exits in 2013 and a three-year major title drought but they managed to remain in the top 5 of the world ranking.

In early 2015, Tan announced his resignation from the Badminton Association of Malaysia. Koo and Tan announced that they are coming out of retirement and try to qualify for the 2016 Rio Olympics before they call it quits for good. They are currently sponsored by Seri Mutiara Development Sdn Bhd They have achieved some breakthroughs this year. Winning the Russian Open, Yonex Dutch Open and finishing runners up in the Thailand Open. They have also made it to 2 Super Series quarterfinals in Australia and Korea.

Tan and Koo had a string of good runs to finish in 11th place in the 2016 race to Rio Olympics standings. However they did not manage to qualify because they were ranked behind fellow Malaysians Goh V Shem and Tan Wee Kiong.